Fix bexti description (#1755)

Author: tomeksowi

src/b-st-ext.adoc

@@ -1282,7 +1282,7 @@ Encoding (RV64)::
 ....
 
 Description::
-This instruction returns a single bit extracted from _rs1_ at the index specified in _rs2_.
+This instruction returns a single bit extracted from _rs1_ at the index specified in _shamt_.
 The index is read from the lower log2(XLEN) bits of _shamt_.
 For RV32, the encodings corresponding to shamt[5]=1 are reserved.